Dichlorodifluoromethane is a colorless gas usually sold under the brand name
Freon-12, and a chlorofluorocarbon halomethane used as a refrigerant and
aerosol spray propellant. Complying with the Montreal Protocol, its manufacture
was universally banned in 1996 due to concerns about its damaging impact to
the ozone layer. Its only allowed usage is as fire retardant in submarines and
aircraft. It is soluble in many organic solvents. Dichlorodifluoromethane was
one of the original propellants for Silly String. R-12 cylinders are colored
white. Preparation
It can be prepared by reacting carbon tetrachloride with hydrogen fluoride in
the presence of a catalytic amount of antimony pentachloride:
CCl4 + 2HF → CCl2F2 + 2HCl This reaction can also produce
trichlorofluoromethane, chlorotrifluoromethane and
tetrafluoromethane. History
Charles Kettering, vice president of General Motors Research Corporation, was
seeking a refrigerant replacement that would be colorless, odorless, tasteless,
nontoxic, and nonflammable. He assembled a team that included Thomas Midgley,
Jr., Albert Leon Henne and Robert McNary. From 1930 to 1935 they developed
Dichlorodifluoromethane, trichlorofluoromethane,
Chlorodifluoromethane, Trichlorotrifluoroethane, and
Dichlorotetrafluoroethane, through Kinetic Chemicals which was a joint
venture between DuPont and General Motors.
Use as an aerosol The use of chlorofluorocarbons as
aerosols in medicine, such as USP-approved salbutamol, has been phased
out by the U.S. Food and Drug Administration. A different propellant
known as hydrofluoroalkane, or HFA, which is not known to harm the
environment, was chosen to replace it. Retrofitting
R-12 was used in most refrigeration and vehicle air conditioning applications
prior to 1994 before being replaced by 1,1,1,2-tetrafluoroethane, which has an
insignificant ozone depletion potential. 1992–1994 was the time period in which
automobile manufacturers started using R-134a instead of R-12. When older units
leak, retrofits to R-134a are recommended. Retrofit to R-134a requires
complete flushing and filter/dryer replacement to remove the mineral oil.
Mineral oil used for R12 is not compatible with R-134a. Some oils
designed for conversion to R-134a are advertised as compatible with residual
R-12. New rubber hoses which are R-134a compatible may be needed for the same
reason. In systems where R-134a is not
practical, an R-409A blend may be directly added to an R-12 system without
changing the oil, although a filter change is always recommended. R-409A
usually runs on the low-pressure side of the system at 12 pounds per square inch,
while R-12 usually runs on the low side at 10 psi. R-409A runs at higher
pressures and is less efficient but works quite well. Manufacturers
recommend that the existing R-12 charge be recovered. However, as the two
refrigerants are soluble in the same mineral oil, there are no complications
associated with mixing them. Note that the instructions for R-409A recommend
charging a system with liquid to keep the mixture ratios constant. The
constituents have varying vapor pressures.
Gallery References
External links NOAA/ESRL CFC-12 global measurements
International Chemical Safety Card 0048 Overview of Freon-12 and some of its
environmental problems MSDS at Oxford University
Thermochemistry data at chemnet.ru IR absorption spectra
CDC - NIOSH Pocket Guide to Chemical Hazards